1. Field of the Invention
The present invention relates to the fabrication of integrated circuits and to the fabrication of photolithographic reticles useful in the manufacture of integrated circuits.
2. Background of the Related Art
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's fabrication plants are routinely producing devices having 0.15 μm and even 0.13 μm feature sizes, and tomorrow's plants soon will be producing devices having even smaller geometries.
The increasing circuit densities have placed additional demands on processes used to fabricate semiconductor devices. For example, as circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Reliable formation of high aspect ratio features is important to the success of sub-micron technology and to the continued effort to increase circuit density and quality of individual substrates.
High aspect ratio features are conventionally formed by patterning a surface of a substrate to define the dimensions of the features and then etching the substrate to remove material and define the features. To form high aspect ratio features with a desired ratio of height to width, the dimensions of the features are required to be formed within certain parameters, which are typically defined as the critical dimensions of the features. Consequently, reliable formation of high aspect ratio features with desired critical dimensions requires precise patterning and subsequent etching of the substrate.
Photolithography is a technique used to form precise patterns on the substrate surface and then the patterned substrate surface is etched to form the desired device or features. Photolithography techniques use light patterns and resist materials deposited on a substrate surface to develop precise patterns on the substrate surface prior to the etching process. In conventional photolithographic processes, a resist is applied on the layer to be etched, and the features to be etched in the layer, such as contacts, vias, or interconnects, are defined by exposing the resist to a pattern of light through a photolithographic reticle having a photomask layer disposed thereon. The photomask layer corresponds to the desired configuration of features. A light source emitting ultraviolet (UV) light or low energy X-ray light, for example, may be used to expose the resist to alter the composition of the resist. Generally, the exposed resist material is removed by a chemical process to expose the underlying substrate material. The exposed underlying substrate material is then etched to form the features in the substrate surface while the retained resist material remains as a protective coating for the unexposed underlying substrate material.
Photolithographic reticles typically include a substrate made of an optically transparent material, such as quartz or fused silica (i.e., silicon dioxide, SiO2), having an opaque light-shielding layer of metal, or photomask, typically chromium, deposited on the surface of the substrate. The light-shielding layer is patterned to correspond to the features to be transferred to the substrate. Generally, conventional photolithographic reticles are fabricated by first depositing a thin metal layer on a substrate comprising an optically transparent material, such as quartz, and depositing a resist layer on the thin metal layer. The resist is then patterned using conventional laser or electron beam patterning equipment to define the critical dimensions to be transferred to the metal layer. The metal layer is then etched to remove the metal material not protected by the patterned resist; thereby exposing the underlying material and forming a patterned photomask layer. Photomask layers allow light to pass therethrough in a precise pattern onto the substrate surface.
Conventional etching processes, such as wet etching, tend to etch isotropically, which can result in an undercut phenomenon to occur in the metal layer below the patterned resist. The undercut phenomenon can produce patterned features on the photomask that are not uniformly spaced and do not have the desired straight, vertical sidewalls, thereby losing the critical dimensions of the features. Additionally, the isotropic etching of the features may overetch the sidewalls of features in high aspect ratios, resulting in the loss of the critical dimensions of the features. Features formed without the desired critical dimensions in the metal layer can detrimentally affect light passing therethrough and result in less than desirable patterning by the photomask in subsequent photolithographic processes.
Plasma etch processing, known as dry etch processing or dry etching, provides an alternative to wet etching and provides a more anisotropic etch than wet etching processes. The dry etching process has been shown to produce less undercutting and improve the retention of the critical dimensions of the photomask features with straighter sidewalls and flatter bottoms. In conventional dry etching processing, a plasma of etching gases is used to etch the metal layers formed on the substrate.
However, dry etching may overetch or imprecisely etch the sidewalls of the openings or pattern formed in the resist material used to define the critical dimensions of the metal layer. Excess side removal of the resist material results in a loss of the critical dimensions of the patterned resist features, which may correspond in a loss of critical dimensions of the features formed in the metal layer defined by the patterned resist layer. Further, imprecise etching may not sufficiently etch the features to provide the necessary critical dimensions. Failure to sufficiently etch the features to the critical dimensions is referred to as a “gain” of critical dimensions. The degree of loss or gain of the critical dimensions in the metal layer is referred to as “etching bias” or “CD bias”. The etching bias can be as large as 120 nm in photomask patterns used to form 0.14 μm features on substrate surfaces.
Additionally, the metal layer, particularly the surface of the metal layer, may incorporate contaminants, such as oxygen and nitrogen, which are more sensitive to etching radicals than the metal layer alone, and can result in the loss of critical dimensions. Also, an anti-reflective coating (ARC) may be deposited on the metal layer to improve the precision of a photolithographic process for patterning a photoresist layer and then etching the metal layer. However, the anti-reflective coating may comprise inorganic matter, and similar to the contaminated metal layer described above, become overetched and result in the loss of critical dimensions.
The loss or gain of critical dimensions of the pattern formed in the metal layer can detrimentally affect the light passing therethrough and produce numerous patterning and subsequent etching defects in the substrate patterned by the photolithographic reticle. The loss or gain of critical dimensions of the photomask can result in insufficient photolithographic performance for etching high aspect ratios of sub-micron features, and, if the loss or gain of critical dimensions is severe enough, can also result in the failure of the photolithographic reticle or subsequently etched device.
One solution to preserving the critical dimensions of a feature is to use processing gases containing passivating materials, such as hydrocarbons, which may form polymeric deposits on the sidewalls of features and prevent overetching. However, polymer-forming compounds may deposit on chamber components and become a source of particulate matter in the processing chamber. Particulate matter may deposit on the substrate surface and detrimentally affect the etching process as well as subsequent processing.
Therefore, there remains a need for a process and chemistry for etching a metal layer on a substrate, such as a reticle, which produces a pattern with desired critical dimensions in the metal layer.